Foaming occurs in many processes in the chemical, food preparation, petroleum, and other industries. Often a foam can overflow the vessel in which a foaming liquid is confined, clogging overflow lines or relief valves, and sometimes exposing operating personnel to hazardous liquids and vapors. Even when confined to the vessel, foams take up volume and thereby subtract from productive capacity.
Foaming is a problem with aqueous processes, such as paper pulping, emulsion polymerization, and the like. Foaming is also a problem with processes involving liquid hydrocarbons, such as petroleum refinery processes, such as delayed coking, asphalt processing and the like; wellhead separation of gas from oil; in hydraulic and lubricating processes in general; and the like.
To avoid foaming, or ameliorate the effects of foaming, foam suppressant compositions are often added to the foaming, or potentially foaming, liquid. A composition added to a liquid before it foams, to prevent excessive foaming, is commonly termed an antifoam. A composition added to an already foaming liquid, to knock down the foam, is commonly termed a defoamer. The term foam suppressant composition is used herein to encompass both these terms, i.e., antifoam and defoamer.
It is known to add small amounts of various silicone compositions to either aqueous liquids or hydrocarbon liquids to suppress foam.
U.S. Pat. No. 3,445,839 discloses an aqueous defoaming composition consisting essentially of a polydimethylsiloxane fluid, a resin composed of (CH.sub.3).sub.3 SiO.sub.1/2 units and SiO.sub.2 units, and a silica aerogel.
U.S. Pat. No. 3,691,091 discloses a silicone emulsion, for defoaming aqueous liquids, in which the silicone consists essentially of a polydimethylsiloxane fluid, silica, and an organosilicon compound or oligomer containing alkoxy and/or silanol groups.
U.S. Pat. No. 4,082,690 discloses a silicone composition, useful for reducing foam in non-aqueous liquids, consisting essentially of a polydimethylsiloxane, a siloxane resin composed of (CH.sub.3).sub.3 SiO.sub.1/2 units and SiO.sub.2 units, and a hydrocarbon solvent.
Other foam suppressant compositions are directed to both aqueous liquids and hydrocarbon liquids. To be useful in both said liquids, it is desirable that a foam suppressant composition be stable upon emulsification, and also stable upon dispersion in a hydrocarbon solvent, in order to facilitate foam suppression.
U.S. Pat. No. 4,145,308 discloses foam suppressant compositions, useful in both aqueous and hydrocarbon liquids, consisting essentially of a polydimethylsiloxane, a silicone resin composed of R.sub.3 SiO.sub.1/2 units and SiO.sub.2 units wherein R is a monovalent hydrocarbon radical, and fume silica or precipitated silica.
U.S. Pat. No. 3,383,327 discloses a foam control agent prepared from a polydiorganosiloxane fluid, silica, and an hydroxylated polydimethylsiloxane.
While these approaches of the prior art produce foam suppressant compositions, certain problems remain. Relatively high silica levels, chemical treatment of the silica, and relatively high resin contents tend to increase the cost of manufacture of said compositions. Additionally, high silica levels make dilution of the foam suppressant composition with a hydrocarbon solvent difficult, as the silica has a tendency to precipitate upon such dilution and thus the silica can become ineffective in the suppression of foam.
None of the above patents teaches the use of two trimethylsiloxy-ended polydimethylsiloxane fluids of differing viscosities, in combination with a silica and a siloxane resin. In fact, U.S. Pat. No. 3,691,091 teaches that if a second trimethylsiloxy-ended polydimethylsiloxane fluid is substituted for the hydroxy or alkoxy substituted organosilicon compound or oligomers of the compositions of said patent, the result of said substitution is a composition that is not flowable, and does not defoamm a foaming liquid.